Electric welding control with phase shift



y 95 s. c. ROCKAFELLOW 2,

ELECTRIC WELDING CONTROL WITH PHASE SHIFT Filed Aug. 9, 1950 LI L2 HEAT TIME 60 F q l g Ll HI L 1 l STUART C. ROCKAFELLOW Snventor Patented July 29, 1952 OFFICE ELECTRIC WELDING CONTROL WITH PHAS E SHIFT Stuart C. Rockafellow, Farmington, Mich., assignor to Robotron Corporation Application August 9, 1950, Serial No. 178,463

current supplied to the welding transformer. This latter is preferably accomplished by means of phase shift.

In presently known systemsaccomplishing this general purpose, the control is subject to variations due to voltage variations and transient conditions. This obstacle has, in the past, made necessary many additional control units to compensate for these variations. Particularly, me-

chanical devices for accomplishing the present purposes have been suggested but they are insufliciently flexible to meet the present demands for accurate control of short periods of time, such as one cycle of 60 cycle current.

This invention meets these problems by using a simple, yet accurate, electronic timing circuit controlling the current fed to a phase shift network. Means for correcting for the power factor of the welding load are incorporated in the phase shift network along with the conventional phase shifting means. The current pulses for the timing circuit are phase shifted through this net work and fed'to the gridsof firing tubes. The pulses to the grids of the firing tubes overcome negative bias and allow the firing tubes to fire at the correct instant. The firing tubes allow the ignitrons to fire in the conventional manner. 1

In the drawings:

Figure 1 shows diagrammatically the circuit of the system.

. Referring now to the figure, a power source of any suitable type is indicated at L1 and L2. This .source energizes the primary I of a transformer lT. Said primary energizes in a conventional manner the secondary 2 of said transformer and this provides the power source for the timing portion of the circuit. At 8 and 9 are indicated small I type thyratron tubes of conventional type each having an anode and cathode, heater, control grid and shield grid. Either of the grids may be used as control elements in this type of tube.

15 Claims. (Cl. 315-197) with respect to each other, and said parallel circuit is then connected at one end through the switch M to one side of the secondary 2 and at its other end through the transformer 2T to the other side of the secondary 2. Also connected across the terminals of the secondary 2 and in series with each other are the capacitor 3', the resistance 4, and the primary winding 5 of transformer-4T. The secondary winding 6 of transformer AT is connected at one end to the oathode of tube 8 and at the other=end through a resistance! to the shield grid of tube 8. The secondary l6 of transformer 4T is connected at one end to the cathode of tube 9 and at the other end through a resistance H to the shield grid of tube 9. The purpose of these circuits is g to impose a small amount of leading potential .onto the shield grids of the tubes 8 and 9. This potential allows these tubes to fire on the forward portion of the wave and this will cause a substantially complete wave form to flow through the phase shift network. This is obtained by.

having enough leading current introduced into the primary 5 of the transformer 4T by. the capacitor 3and the resistor 4 to cause both tubes '8 and 9 to conduct very close to the zero point of the wave. Placed in parallel with the circuit comprising the tube 9 and the switch I 4 is the primary winding 28 of the transformer 5T. Its secondary 2| is connected through the resistance 20 to the control grid of the thyratron 9 and through the, resistance 24 to the cathode of the thyratron 9. This circuit is so phased that when the anode l8 of the thyratron 9 is at positive potential the secondary 2| will supply negative pulses to the control grid [9 and thereby tend to hold the tube 9 non-conducting. To further protect the tube 9 from firing before necessary, the secondary winding 21 is also provided for the transformer ST and each of its ends is connected through resistances 25 and 26 to the rectifier 23 which in turn is connected to the cathode side of the capacitor 22. The other side of said capacitor, is connected to. the grid l9. Thus, pulses through the primary 28 of the transformer ST in both directions will maintain a constant,

' though small, negative potential on the grid side These tubes are connected in parallel, but with their respective anodes and cathodes reversed of the capacitor 22. 1

The thyratrons 8 and 9,.just described, are the firing tubes and control the positiveand negative pulses through the primary winding I3of the transformer 2T. These firing tubes in turn are controlled by the thyratrons l0 and I l as will now be explained.

For convenience in reference, the side of the secondary 2 which is connected to the cathode of the tube 8 will be referred to as the left side of said secondary and the side which is convided with a cathode, an anode, a heater, con? trol grid and shield grid. The cathode and shield I grid of the tube It is connected to theleftiside' of the secondary winding 2 of transformer IT. The anode of the tube It is connectedthrougha time controlled circuit, comprising the capacitor 33 and the variable resistance 3'! in parallel, to

other side of the secondary 2 of transformer IT.

The control electrode 33 of tube I3 is connected through the resistor 34 to the cathode of the tube .23 and is also connected through the capacitor 33 and the resistor 32 to the left side of the secondary 2. The resistor 3| is connected .at-one side to a point between the resistor 32 and the capacitor 33"and at its other side through the primary I3 of transformer 2T-to the rightv side of the-secondary 2 of transformer IT. Thispro- .vides a voltage divider network to holdthe potential on the electrode 38 within the proper limits.

1 The right hand side of secondary Zis connected through a resistance 29 to a point intermediate the ends of. the primary -28 of transformerBT. Tube H has its shield grid andcathode connected to the right side of the secondary 1 2 of transformer 5T and its anode is connected through a timing network, comprising the capacitor 4| and variable resistance 42 connected in parallel to the left sideofthe secondary 2. The anode of said tube, II is also connected to the control grid I5 of thetube 3,. and theanode of the tube, In .is connected; to the .control grid .39 of the tube II The-secondary 46 of the transformer 2T-is connected at its one side to 'a pair ofv variable resist- ,ances -43 and 44 in series with each othera'ndits other side to one side of the capacitor 4.5. The other side of said capacitor is connected to the free end of variable-resistance 44. A point'interv .rnediate theends of the secondary 46 is connected to one side of the primary 4? of transformer 3T andthe .other side of said primary is connected ata point between the capacitor .45 and the resis- Itor -44. a

. The firing tubes 55 and 53 are of the typeconventionally used tostart ignition type tubes. The tube 55 has in its grid circuit a constant voltage source 53 connected in series with the secondary 48 of thetransformer 3'1 and the resistance .43. The tube Sfihas in its grid circuit a-constant potentialfiI connected in series with the secondary 52 of transformer 3T and the resistance 53. connected in, a conventional manner to igniter The firing tubes 55 and 56 are circuits of the ignitrons, and these ignitrons are connected in a conventional backet'oback circuit in a circuit including the welding transformer I-II and the power source LI.--L2.

' Operation .With the switch I 4 open, only the tube II] is conducting; This charges the capacitor 36 with negative potential at its anode end and said potential is also placed on the control grid 39'of the tube II ,thus rendering said tube II non-conductive. At this time also both .uegativ'e and positive pulses arepassing through thejprima ry 4 28 of the transformer ST and through the action of the secondary winding 2I and 21 a negative potential is being held on the control electrode I3 of the tube 9. Also, pulses from the left side of secondary 2 of the transformer IT are passing through the resistance 32 and charging the capacitor 33, said capacitor circuit being completedby grid: to cathode conduction in tube II'I and through the primary 28 "of transformer ST and the primary I3 of transformer 2T to the right side of the secondary tube. Thus, with switch I4 in open position the condition of the timing cir- .cui:t :is as follows:

'Tube I3 is conducting and charging capacitor 3'6 which holds tube I I blocked. Capacitor 33 is charging, :but to such polarity that it does not interfere with the conduction of tube I 3. Capacitor 22 is also charging and a negative potential is heldion the control electrode of the tube 9. No current is passing through the primary I3 excepting thesmallamount permitted by the grid to cathode conduction in tube II] which is 'negligible insofaras the firing tubes 55 and .55 are concerned, and thus the said firing tubes and the ,ance 3?. of time continues to hold the tube I I blocked and lassocia'ted ignitrons are inactive.

NVhe'n the switch -I l..is closed, the .thyratron .3

will. conduct .on one-half cycle, which may befor convenience termed the "-positive half lcycleand 'on the electrode. 13. However, .as}soon as the tube 8 conducts, it effectively shorts the positive half cycles through said tubes and prevents their passing through .the primary 2.8 of the transformer 5T. Thisineffect, reverses the polarity of thetransformer 5T so that when the anode of tube 9 becomes positive on a negativehalf cycle,

its grid is also positive and permits the tube to conduct and thus it passes the negative half cycle. Thus, both negative and positive Ihalf cycleswill pass through the primary I3 of transformer'2T and because of the leading potential imposed upon the shield, grids of tubes Band 9 as above described, the primary I3 will be subjected to .substantiallythe full wave provided by the power source.

With the'shorting of the positive half vcycle through the tube 8, the capacitor 33 ceases charging and acts to place a negative potential on the control grid 38 of the tube It). This blocks the tube It and permits the .capacitor 36 to commence discharging through the variable resist.-

Said capacitor 33, however, for a period hence the tubes 8 and 9 continue to conduct. This is the heat time and will continue for so long as the capacitor 36 holds the tube I I blocked.

When capacitor 35 has drained sufficiently that the tube I I. will commence to conduct; such conduction will start to charge the capacitorAI, and simultaneously impose a negative potentiaLonto the. control electrode I5 of the tube 8., This blocks the tube 8 and re-establishes a potential level is'the off-time."-

When it does reach such a'level, the tube 8 will resume conduction and the cycle will repeats. It'will be evident that so long as the switch- M remains closed, the above described cycle 1 will continue to' repeat and :the

duration of heat time and offtimewill be "controlled byth'e settin'g of the' variable'resist- -an'ce 31 and-"respectively. I Y w The full wave current passing through the primary [3 will induce a similar full wave in the secondary 46 and the phase thereof may be shifted according-"to presently known principles the larger 'resistanc'eis used for phase shift and is' of such 'a value; according topres'ently' known principles; as to give 180 degrees shift, The "smaller var'iable resistance is used asa power factor control and is'adjuste'd, with the'other variable resistor completely closedj-soas to start the final conduction through'the firing'tubes '55 and 56 at that point of the wave which is at the power factor of the welding transformer and load.

The phase shifted full wave current from the timer now passes through the primary 4! of the transformer 3T. The constantnegative sources 50 and 51 provide a bias on the firing tubes 55 and 5B and keep said tubes from firing until said bias is overcome by the energization thus mentioned of the primary 41 acting through the secondaries 48 and 52. Thus, tubes '55 and 56 will fire, and fire the ign'itrons 60 and SI in a conventional manner at the point at which the negative purpose.

' I claim:

1. An electronic control circuit comprising in combination: a source of alternating potential;

a pair of electric discharge devices, each having an anode, a cathode, a shield grid and a con- 'trol grid; means imposing onto the control grid of each electric discharge device a leading potential sufficient to cause said electric discharge devices to conduct at a point near the zero point of the wave form emanating from said source; means controlling the periods of conduction and non-conduction of said electric discharge devices; a phase shift circuit and means vsupplying the output of said discharge devices to said phase shift circuit; a load and means connecting. said load to a source of alternating potential; a pair of ignitron type electric discharge devices connected in back-to-back relationship and in series with said load circuit and'a-pair of ignitron controlling electric discharge devices and means connecting same into the igniter circuits of said ignitron electric discharge devices; means providing a constant negative bias potential on each of the control electrodes of said ignitron controlling electric discharge devices and means within said circuits of said electrodes and energized by said phase shift circuit to overcome said constant bias when the pulse wave in said phase shift circuit reaches a predetermined potential:

whereby the conduction and non-conduction of;

20f'a switch, a phase-shiftcircuit receiving the outjaycathode and a control electrode and being said ignitrons is controlled by said timing cir cuit:and the precise point on the wave-form at which conduction by 'thefiignitrons is started i and determined by the phase shift circuit;

'Z'.? 2'.' .AIi"e1eCtI'O1liC control circuit for opening and closing a Switch comprising in combination: a-source of alternating potential;- a pair of electric discharge device's, each having an anode,.-a cathode, a shield grid and a control grid; means *connecting said electric discharge devices to said 'sourcein parallel with each other and of oppdsite polarity with respect to each other; means imposing onto'the control grid ofreachfielectric di'scharge' device a I leading potential suflicient to cause said electric discharge devices tc conduct at a pointnear thez'ero point'of the waveformemanating from said source; means conconduction of said electric discharge devices;

put of the saidelectric discharge devices and means e'nergized'by said phase shift circuit'for "opening and'closing said switch, said means including means providing a constant potential j normally holding 'saidswitch open and including f fu'rther' means so 'adaptedthat energitation thereof by the said phase shift circuit overcomes said constant potential at a precisely controllable time and switch v '3'. Anelectronic timing control circuit for thereby elfects the closingjof said opening and closing a switch" comprising in combination: a power source; a first electric discharge device having'an anode,-*a control'electrode and a cathode; a transformer having primary'and secondary windings and said electric "discharge device being connected at its anode and itscathode in series with the primary winding of said transformer and said power source; a second electric discharge device having an anode,

connected by its anode and its cathode in parallel 'with' said first electric discharge device and in opposite polarity with respect thereto; a second {transformer having a primary winding and a pair offsecondary windings, said primary winding being connected in parallel with said second electric dischargedevice, one of said secondary windings being connected at its one end to the control electrode of said secondelectric discharge "jdevic'e and at its other end to the cathode there- 'of ,jand the other of said secondary windings being connected atjeach of its ends through a pair of resistances and a rectifier to one side of al capacitor, and the other side of said capacitor being connectedjt'o said control electrode of said second electric discharge device; a phase shift circuit including the secondary winding of 'said first'transformerandmeans energized by said phase shift circuit to actuate said switch. M An electronic timing control circuit for opening and closing'a switch comprising in combination: a power source; a first electric dis- -=charge device havingan anode, a control electrode and av cathode; atransformer having primary and. secondary windings, said electric 'discharge device being connectedv at its anode and its cathode in series with the primary winding of said transformer and said power source;

discharge device 'and at .its :other -tendf-to the cathode thereof, and; the other: ofrfsaid secondary windings being connectedzza't :each: of its ends throughta pair'of resistances and a Lrectifierto 1 one :side :of a first capacitor,-. and the other side -of said capacitor being connected to:said:control a electrode. or said secondelectric discharge device;

Ethi'rd andfourth electric discharge devicesveach having ananode, a cathode andaa-icontrolielec- .trode; means connecting the rcathode of said third electric discharge devicetowthe same side of said, power source as the cathode of said :first electric discharge device; -:a circuitlincluding Ea second capacitor and-:a resistor in parallel with each 'otherconnecting the anode of said-third" 'ielectriczdischargewclevice to the 'opposite'side; of

said power source and -means connecting ;the anode of the third electric dischargedevice also to the control electrode of thefourth :electric discharge device; means-connecting the cathode of said .fourth electric discharge device to said other side of said power source and :a ,c ircuit including a third capacitor and a resistor -in parallel connecting the anode. of saidfourth electric discharge device to the same side of gsaid power source-as is connected the cathodeofthc .iondarywindings. andimeans energized .from. said '=.phase shift .circuit zconnected to the primary winding of said :second'ltransformer; an electric .network' controlling said "switch and including :an electrical discharge device having va control Y electrode; means providing a :constant bias, said control electrode opposingconduction by theoelectric discharge device;-means connecting thesecondary "winding oil-said second transformerinto the circuit of said lcontrolelectrode in such:plar- Y ity-thatra, pulse induced therein by a, pulse;in

first electric discharge .device; .and 7 means also ing; a first transformer having a primary winding and .a secondary Winding; an electricalnetwork supplying a substantially full wave of. alternating electric potential to saidprimary winding .and said network including means initiating, and terminating the'supplying of said wave to saidjfpflvmar-y winding; a pair of variable resistors and a capacitor in series with each 'other'and means connecting same invseries with said'secondary winding; a second transformer having a primary winding and means connecting onerend of said primary winding to a point between said capacitorand one of the'said variable. resistors nearest thereto; and meansconnectingtheother end of saidlast .name'd primary winding toithe said secondary'winding of said first transformer ata point intermediate the ends thereof; an electrical network connected with said secondary winding or said-second transformer for opening and closingsaid switch.

{6."In an electricaltiming and 'control cir cuit for use'with "a source of electrical potential "and -'a*-load, the combination comprising: a first transf-ormer havingprimary and secondary windings,

'anelectrical"network including .timing control ,meansfor delivering substantially full wave pulses to the primary winding of said transformer: and

. meansiincluded therein for automatically 1 startingaandrstoppingthe. flow of said pulses; aphase shift circuit and' means supplying said 7 "circuit @from the secondary winding. of said-transformer :a-'fsecond transformer including :primaryiand; seconedirection in th'eprimary winding in said-.first transformer will" overcome said bias andzcause the electric :discharge device to conduct; "means-responsive -to:said conductionof said; electric :dis-

charge device; for closing said ;;switch;vlwhereby the time of closingof: said switchmay-.be accurately controlled by .the adjusting of the (phase shift so that a pulse delivered from the 'phase shift 1: network will ;overcome said vconstant, bias 1 at aaprecisely predeterminablepoint: on the waveform supplied by: said source;

7. An electronictiming control circuit for Opening and closing ass/itchcomprising in. combination: a power source; a first electric dischargedevice having an anode, lacontrol-electrode and a cathode; .a iirstgtrans former;having .primary and secondary1win: ding's,- said electric-discharge device being connected at its anode/ and its cathode in serieswith. the primary winding of said trans former. and said power source; a second electric discharge ,devicehaving-anranode, a cathodea-nd a control electrode andbeing connectedbyits anode and its cathode in parallel with said first electric dischargedeviceand in opposite polarity with respect thereto a. second ltransformerfihaving alprimary winding and alpair of seconclary windings, said primary, winding being. connected parallel with said second electric discharge de-' vice, one ofsaidrsecondary windingsbeing lconnected at; its one endto the control electrodoiof said second .electric.discharge devi'cefian'd. at its other end to the, cathodefthere'of; a pair ,of ore sistances connected in series ,1 amectifier; means connecting the free ends of said resistancesflto each respective end of the other of said secondary windings and means connecting the. mutually connected ends of said-resistances through said rectifier to one'sideo'f a first capacitor, and the other side of said capacitor being connected'to said Con?- trol electrode of said second electric discharge device; third and fourth electric discharge devices each having an anode, a cathode and aocontrol electrode; means connecting the cathode of said third electric discharge device to the same side of said power source as the cathode of said first electric discharge device; an adjustable timeconstant circuit connecting the anodeof said third electric discharge device to the opposite sideof said power source and means connecting f the anode of the third electric discharge device also to the control electrode of the fourth'electric discharge'device; means connecting thecathode of said fourth electric-discharge device to'said other side of said. power source and means including'an adjustable time-constant circuit connecting the anode of said fourth electric dis'chargedevice to electric discharge device ;and means energized by said primary of said first transformer for open- 7 ing and closing said switch. 7 I

8. An electric timing control circuit for open-,-

ing and closing a switch comprising in combination: a power source; a first electric discharge device having an anode, a control ele'ctrode and a cathode; a first transformer having primary and secondary windings, said electric discharge device being connected at its anode and its cath-v ode in series with the primary winding of said transformer and said power source; a second electric discharge device having an anode, a cathode and a control electrode and connected by its anode and, its cathode in parallel with said first electric discharge device and in opposite polarity with respect thereto; a second transformer having a primary winding and a pair of secondary wind-h ings, said primary winding being connected in parallel with said second electric discharge de vice, one of said secondary windings being connected at its one end to the control electrode of said second electric discharge device and at its means normally blocking said first time con trollab'lemeans; means for blocking said second time}controllable'means; a'capacitor and means connected from one side of said capacitor-to the same side of said source as the. cathodejjofflthe' first electric discharge device and means con nected from the other side of said capacitor to said means blocking said second time controllable means for energizing same.

9. An electric timing control circuit for opening and closing a switch comprising in combination: a power source; a first electric discharge device having an anode, a control electrode and a cathode; a first transformer having primary and secondary windings, said electric discharge device being connected at its anode and its cathode in series with the primary winding of said transformer and said power source; a second electric discharge device having an anode, a cathode and a control electrode and being connected by its anode and its cathode in parallel with said I first electric discharge device and in opposite polarity with respect thereto; a second transformer having a primary winding and a pair of secondary windings, said primary winding being connected in parallel with said second electric discharge device, one of said secondary windings being connected at its one end to the control electrode of said second electric discharge device and at its other end to the cathode thereof, a pair of resistances connected in series; a rectifier; a first capacitor; means connecting the free ends of said resistances to each respective end of the other of said secondary windings and means connecting the mutually connected ends of said resistances through said rectifier to one side of said first capacitor, and the other side of said capacitor being connected to said control electrode of said second electric discharge device; first and second automatically controllable switches, each having an electrically responsive control means; means con- 10:" necting the input sideof said first automatically controlled switch tothe same side of said power source: as the cathodeof said first electric dischargefdevice; an adjustable time constant circult connecting the output side of said first automatically controllable switch to the other side of said power source and means connecting the output side of said firstlautomatically controllable switch also to the control means of said second I automatically controllable switch; means connecting the input side of said second automatically controllable switch to said other side, of said power source and means including an adjustable time-constant circuit connecting the output side of said second automatically controllable switch to the same side of said power source as is connected the cathode of said first electric discharge device; and means also connecting the output side I of said second automatically controllable switch to thecontrol electrode of the first electric discharge device; a further capacitor and means connecting onef side thereof to the last named side of said power; source and means connecting the s 10. An electronic timing control circuit for openingjand closing a switch comprising in combination: a power source; afirst electric discharge device havingan, anode, a control electrode and a cathode; a first'transformer having primary and secondary windings, said electric discharge device being connected at its anode and its cathode in series with the primary Winding of said transformer and said power source; a second electric discharge device having an anode, a cathode and afcontrol-electrode and being'connectedby its anode and fits cathode in parallel with said, first electric 'disjcharg e device and in opposite polarity witli'respect -thereto; a second transformer having afprima'ry .w'in'ding "and a pair or secondary windings, said primary winding being connected in parallel with said second electric discharge device, one of said secondary windings being connected at its one end to the control electrode of said second electric discharge device and at its other end to the cathode thereof, a pair of resistances connected in series; a rectifier; a first capacitor; means connecting the free ends of said resistances to each respective end of the other of said secondary windings and means connectin the mutually connected ends of said resistances through said rectifier to one side of said first capacitor, and the other side of said capacitor being connected to said control electrode of said second electric discharge device; means controlling the periods of conduction and non-conduction of said electric discharge devices within predetermined time limits; and means energized by the primary winding of said first transformer for opening and closing said switch.

11. Apparatus defined in claim 3 including also means imposing onto the control electrode of each of said first and second electric discharge devices a leading potential sufficient to cause said first and second electric discharge devices to conduct at a point near the zero point of the wave-form emanating from said source. 1

12. An electronic control circuit for opening and closing a switch comprising in combination: a source of alternating potential; a pair of electric discharge devices, each having an anode, a cathode, a shield grid and a control grid; means connecting said electric discharge devices to said source in parallel with each other and of opposite p rity with respect to each other; means'irn' posing onto the control grid of each electric discharge device aleading potential suificient to cause said electric discharge devices to conduct at a point near the zero point ofthe wave-form emanating from said source; means controlling the periods of conduction and non-conduction of said electric discharge devices; 'a first transformer, having its primary winding in the anode cir' cuit of oneof said electric discharge devices; a pair of variable resistors and a capacitor in series with each other and means'connecting same in series with the secondary'winding of" said first transformer; a second tran'sformer'having a primarywinding and meansconnecting one end of said primary Winding to a point between capacitor and" one of the said variable resistors nearest 7 thereto; and means connecting theother end of said last named primary winding to the said sec-' ondary ofsaid firsttransformer' at a'point inter-' mediate the ends thereof; an electrical network --co'nnectedwith said secondary winding of said second" transformer'for opening andclosing said Switch." I

13 An electronic control circuit for opening and closingfa switch comprising-in combination:

a a sourceof"alternating potential; a pair of electric dischargedevices'having an" anode, .a cathode, arshield gridan'd a control grid; means'coritric discharge devices and" means" energized by' said phase shift circuit for opening' andiclo'singL 14-: Anelectroniccontrol circuit for openir'ig and closing a switch, comprisingin-cornbination: a source of alternating potential; atransformer having primary' and secondary windings; means including a pairof electric discharge devices -c'on-' nected to said source in parallel with'each other" and in opposite polarity with respect to each other'energizable from said source providing intermittent timed'pulses to the'primarywinding" of said transformer; a phase'shift circuit including'the' secondary Winding of saidtransfbrmer;

means'energized by said phase shift circuit for opening and closing said switch;

15. 'An electronic control circuit for opening" and closing a switch comprising combination: a source of alternatingpotential; a pair of"'elec-'- tricdischarge devices having 'an' anode; a oath ode; a shield grid anda control grid; means con-- meeting said electric discharge devices to said source in parallelwith each other. and -o'foppo" site polarity with resp'ectto each other; means controlling the periods of conduction and non conduction of said electric discharge devices;'a-

switch; a" phase shift circuit receiving the output of the said-electric discharge devices andmeans energized by said phase shiftcircuit for opening and closing said switch; r 4 V v v STUART C. ROCKALFELLOW- REFERENCES CIT-ED The followingreferences are ofrecord the file of this patent': V UNITED- STATES PATENTS Number Name" Date 1,851,692 Zucker Mar.29,.1932 2,263,773" Gulliksen Nov;25, 1941 2289321" Collom 'July"'7', 1942 2;s7o ,071 Poole Feb. 20; 1945' 2,473,238"

Bivens 'June'1'4; 1949 

